[MITgcm-devel] seaice

[Jinlun Zhang]

Martin Losch wrote:

> Hi there,
> I have (again) questions about the seaice-pkg. I am working on a  
> 2x2*cos(phi) deg quasi global (no arctic) model approximately 30 to  
> 50km resolution in the Weddell Sea (and Ross Sea). I am forcing with  
> the CORE fields (with daily winds, tair and humidity), but my  
> problems probably not connected to the forcing. I use the exf-pkg and  
> kpp and seaice (SEAICE_MULTILEVEL undefined).
> My basic problem is that there is not enough sea-ice in the Weddell  
> Sea (and the Ross Sea, but that's a different story). One of the  
> reasons is that the ACC "breaks" through the South Scotia ridge (the  
> extension of the Antarctic pensinsula) into the Weddell Sea and  
> directly imports warm water instead of staying further north and  
> introducing the warm water further east into the Weddell Gyre. As a  
> consquence (or a cause?) there is no Weddell Gyre to speak of and too  
> little ice (the distribution is OK, but the thickness is too small  
> towards the north-east of the Weddell Sea).

Hi Martin,

The MITgcm sea ice model  is the one I used/tested based on POLES 
forcing 
(http://psc.apl.washington.edu/POLES/model_forcings/ModelForcings.html) 
for Arctic and NCEP/NCAR forcing for both polar oceans. Some tuning 
involved. If you got too little ice in the Weddell/Ross sea, it is 
likely that the CORE forcing (dynamic or thermal) is significantly 
different.

> The funny thing is that, when I turn off the seaice-model, things  
> look much better (except that the surface is now too cold, which I  
> would have expected in the absence of ice growth): I have a strong  
> Weddell Gyre and the ACC stays completely north of the Weddell Sea as  
> it should. The same is true, when I use ncep heat fluxes and surface  
> field restoring (as in global_ocean.90x40x15). So topography (which  
> was my first guess) cannot be the reason for this behavior, but it is  
> most likely the seaice that makes the Weddell Gyre disappear and lets  
> the ACC into the Weddell Sea. Candiates are:
> 1. not likely: buoyancy fluxes: I don't see how the buoyancy fluxes  
> can cause this problem. I have SEAICE_EXTERNAL_FLUXES defined, so  
> that exf does all the work over open water.

If you found that surface stress does not affect the ocean cir., then it 
could be due to buoyancy fluxes.

> 2. more likely: momentum forcing: But ... I tried a physically  
> sensible stress formulation (seaice_concentration*ocean_ice_stress +  
> (1-seaice_concentration)*ocean_air_stress, see ostres.F with  
> SEAICE_TEST_ICE_STRESS_1 defined); and I tried using only  
> ocean_air_stress as precomputed by exf_bulkformulae (no influence of  
> ice on stress, the default). Both give similar results, which I don't  
> understand.

Try to just use 0*ocean_ice_stress +  (1-0)*ocean_air_stress and see 
what happens. This would exclude ice dynamics effect on ocean cir. if 
you suspect that ice dynamics is the trouble maker.

>
> In order to understand what I am doing when I mess up the code, I  
> need to know what the different fields are. As far as I can see, the  
> seaice-velocities UICE and VICE have three time levels, with UICE(:,:, 
> 1,:,:) being the most current, and UICE(:,:,2,:,:) being the previous  
> timestep and level 3 probably some intermediate step/auxiliary  
> variable used in lsr.F. But what are UICEC and VICEC, in what sense  
> are they different from the first time-level of UICE/VICE?
>
> The SEAICE_ORIGINAL_BAD_ICE_STRESS in ostres.F uses a mixture of UICE 
> (:,:,1,:,:), UICE(:,:,3,:,:) and UICEC to compute the stresses, which  
> I do not understand at all. Which is the ice-velocity to use for  
> computing ocean-seaice stresses?

UICE(1) is the one,  ice velocity, others are all intermediate 
quantities for numerical reasons (or stability). Since UICEC is used in 
the dynamics solver, it shows up in ostres.F for an exact calculation of 
stress.

>
> Puzzled,
>
> Martin
>
>
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-- 

Jinlun Zhang
Polar Science Center, Applied Physics Laboratory
University of Washington, 1013 NE 40th St, Seattle, WA 98105-6698

Phone: (206)-543-5569;  Fax: (206)-616-3142
zhang at apl.washington.edu
http://psc.apl.washington.edu/pscweb2002/Staff/zhang/zhang.html